Exact drag increment.
What type of transform are you using, moving, roatating scaling?
How are you "dragging", from the transform manipulators?

If so either use hotkeys instead, G for moving, Rfor rotating, and S for scaling. That will allow you to input an axis constraint with X,Y or Z and then iput a distance, factor, or angle afterwards.

Just type the value and look in the bottom left of the 3D View header. + and - will toggle positive or negative values. You can even input simple math expressions by entering formula mode by first typing an asterisk (*), then typing down any math operations desired.

If you still want to use the transform manipulators then the only way to introduce precise values is either from the operator properties on the bottom left of the Toolshelf (T key) or from the operator popup by pressing F6 after transforming.